Motors are often used for providing lifting or hoisting power for a load. These loads are often held by a mechanical brake when stopped. Several problems arise in controlling such a load. First, there is a need to bring the load to a stop at a precise height, in the case of an elevator for example. Second, there is a need to detect any brake slippage, which can be the result of mechanical wear on the brake or other factors.
U.S. Pat. No. 5,457,372, discloses a braking method for stopping a hoist motor in which there is a power sensing circuit for sensing the power applied in stopping a load and storing a sampling signal. The basic braking method uses DC current (zero frequency current) that is injected into the stator windings of an AC motor. This produces a stationary magnetic field in the motor air gap to oppose rotation. This basic stopping technique is modified by utilizing the sampling signal. This method does not address the problems of mechanical wear on the brake as discussed above.